The non-equilibrium segregation of boron on original and moving austenite grain boundaries

The influences of boron addition, testing temperature and holding time were studied on the kinetics of austenite recrystallization and on the extent of boron segregation in a series of HSLA steels containing B, Nb, and Nb + B. By means of particle tracking autoradiography (PTA), it is shown that recrystallization, quenching, and deformation all induce the non-equilibrium segregation of boron to austenite grain boundaries. The observations indicate that boron segregation decreases with quenching temperature and that deformation induces segregation on the original boundaries, which persists until recrystallization starts. The non-equilibrium segregation of boron on moving boundaries is described, which decreases to the equilibrium state of segregation when recrystallization is complete, at which point it is no longer detectable by the PTA technique. Finally, a comparison is made between the segregation behaviors of the B and Nb-B steels under different experimental conditions. The non-equilibrium segregation of boron during quenching is interpreted in terms of the solute-clustering model proposed by Aust and coworkers. Boron segregation on moving boundaries is attributed to the time required by a boundary to incorporate a dislocation into its structure. (C) 2002 Elsevier Science B.V. All rights reserved.